In an effort to be "user-friendly," Windows (and perhaps some other operating systems) hides the most important part of a file name from new computer users: the extension. Okay - we're assuming that the reasoning behind hiding extensions is a "user-friendly" one because we just can't come up with any other reason for hiding them. No harm could ever come from seeing an extension, but plenty could be learned from it. Fortunately you have this article to guide you through some of the most common extensions that you'll run into.
But before you can see file extensions, you need to turn them on. From Windows Explorer, click on the "Tools" menu, and select "File Options." Click the "View" tab and then uncheck the box next to "Hide file extensions for known file types." Click "OK" and you'll notice that the files in Windows Explorer show a dot and group of three letters after their names. That dot and group of three letters is known as an "extension," and the extension explains what kind of file it is.
A file could be a plain text file, an image, a sound, a video, or program. But without seeing the extension, you wouldn't know it unless you double-clicked on it. The following list defines some of the most common extensions that you'll find on your computer.
.au - This extension indicates a sound file. Most sound players will load up and play this kind of file.
.art - This extension indicates an image file that was compressed with AOL (America Online) technology. Both Internet Explorer and the AOL service software can display this kind of file, however if you don't have AOL installed on your system, Internet Explorer will display it.
.avi - This extension indicates a video file playable by most multimedia viewers including Microsoft's Media Player.
.bmp - This extension indicates another image file that might have originated from Windows Paint program.
.dll - This extension indicates a Dynamic Link Library which may contain additional programming code for software. Many different programs often share Dynamic Link Libraries and you'll find a bunch of them in the Windows/System directory (but don't ever delete them)!
.exe - This extension indicates a program or an application like Microsoft Word, Internet Explorer, or Outlook Express. Use extreme caution when downloading .exe files from the Internet since malicious programmers like to hide viruses in these types of files.
.gif - This extension indicates another image file and it stands for "Graphics Interchange Format." .Gif files are often smaller than .bmp files (described earlier) and they're commonly found on Internet web pages.
.jpg - This extension indicates yet another image file and it stands for "Joint Photographers Experts Group." Like the .gif file, it's commonly found on Internet web pages, however it's much smaller than both the .gif image and the .bmp image.
.mid - This extension indicates a sound file created with a Musical Instrument Digital Interface. Windows Media Player will open and run these files, however they don't sound like normal .wav or .mp3 files (described later). .Mid files are designed to product synthetic sounds using a computer's sound card.
.mp3 - This extension indicates a sound file that authentically reproduces voice and/or music. Windows Media Player will open and run this kind of file.
.scr - This extension indicates a screen saver file.
.sit - This extension indicates a Macintosh archive StuffIt file. They will not open on a Windows system without a special utility.
.ttf - This extension indicates a font especially designed for use on a Windows system. It stands for "True Type Font."
.txt - This extension indicates a plain text file that can be opened with Notepad.
.wav - This extension indicates a sound file that like the .mp3 file, can be opened with Windows Media Player or Windows Sound Recorder. .Wav files are much larger than .mp3 files.
.zip - This extension indicates a Windows archive WinZip file. They will not open on a Macintosh system without a special utility.